Explorer 39, also known as AD-C (Air Density experiment-C), was a

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scientific Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...

satellite belonging to series Air Density. It was launched on 8 August 1968, join with

Explorer 40 Explorer 40 (or Injun 5), was a NASA magnetically aligned satellite launched simultaneously with Explorer 39 (AD-C) (Air Density experiment) using a Scout B launch vehicle. Mission Explorer 40 was designed to accomplish the following objec ...

, from Launch Complex 5 of the

Vandenberg Air Force Base Vandenberg may refer to: * Vandenberg (surname), including a list of people with the name * USNS ''General Hoyt S. Vandenberg'' (T-AGM-10), transport ship in the United States Navy, sank as an artificial reef in Key West, Florida * Vandenberg Sp ...

, through a

Scout Scout may refer to: Youth movement * Scout (Scouting), a child, usually 10–18 years of age, participating in the worldwide Scouting movement **Scouts (The Scout Association), section for 10-14 year olds in the United Kingdom ** Scouts BSA, sect ...

launch vehicle.

Spacecraft

Explorer 39 was an inflatable sphere, in diameter. It was orbited to make

atmospheric An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

density determinations. The satellite was successfully launched into a nearly polar,

highly elliptical orbit A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity, usually referring to one around Earth. Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used the ...

. It was folded and carried into orbit, together with ejection and inflation equipment, as part of the payload of Explorer 40. Two density experiments were performed.

Launch

Explorer 39 orbited the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

once every 118.20 minutes, at an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

of 80.60°. Its

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

was and

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any el ...

was .

Mission

One involved the study of systematic density variation, and the other was concerned with nonsystematic density changes. The upper atmospheric densities were derived from sequential observations of the sphere by use of an attached 136.620-

MHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...

radio tracking

beacon A beacon is an intentionally conspicuous device designed to attract attention to a specific location. A common example is the lighthouse, which draws attention to a fixed point that can be used to navigate around obstacles or into port. More mode ...

and by optical tracking. The radio beacon ceased transmitting in June 1971. Since that time it has been necessary to rely solely on the

Smithsonian Astrophysical Observatory The Smithsonian Astrophysical Observatory (SAO) is a research institute of the Smithsonian Institution, concentrating on astrophysical studies including galactic and extragalactic astronomy, cosmology, solar, earth and planetary sciences, the ...

(SAO)

Baker-Nunn camera A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. The design was invented by Bernhard Schmidt in 1930. Some notable ...

network for tracking. Explorer 39 has an expected orbital lifetime of 50 years.

Experiments

Nonsystematic Changes of Air Density

This experiment was designed to determine nonsystematic upper atmospheric density changes. The data are derived from studies of the drag on a in diameter low-density sphere caused by short-term differences in solar activity. Density values near perigee were deduced from sequential observations of the spacecraft position using optical (Baker-Nunn camera network) and radio and/or radar tracking techniques. This experiment has determined reasonable density values, and it is capable of yielding long-term atmospheric density values, as Explorer 39 has an expected orbital lifetime of 50 years.

Systematic Changes of Air Density

This experiment was designed to determine systematic changes of air density as a function of altitude, latitude, and time of day, by measuring the drag on a in diameter low-density sphere with ground tracking.

Decay

Explorer 39 reentered the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...

on 22 June 1981.

References

External links

Explorer Series Satellites
Colorado State University
Explorer 39

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...

{{Orbital launches in 1968 Spacecraft launched in 1968 Explorers Program Balloon satellites